Transducer having a backing mass spaced a quarter wavelength therefrom



, Nov. 25, 1969 J. H THOMPSON 3,480,906

TRANSDUCER HAVING A BACKING MASS SPACED A QUARTER WAVELENGTH REFROM Filed Mars 1968 United States Patent 3 480 906 TRANSDUCER HAIIING A BACKING MASS SPACED A QUARTER WAVELENGTH THEREFROM John H. Thompson, Pittsburgh, Pa., assignor to Westing- 5 house Electric Corporation, Pittsburgh, Pa., 21 corporation of Pennsylvania Filed Mar. 13, 1968, Ser. No. 712,829 Int. Cl. H04b 13/02 U.S. Cl. 3408 7 Claims ABSTRACT OF THE DISCLOSURE A transducer for deep sea operation having a front radiating surface with an inertia loading mass extending around the remaining surfaces of the transducer element at a distance d therefrom. The distance d is equal to a quarter wavelength (N4) of the operating frequency of the transducer. The space between the inertia loading mass and the transducer element is filled with a pressure transmitting compliant material.

BACKGROUND OF THE INVENTION Field of the invention This invention in general relates to transducers, and more particularly to a pressure compensated transducer for use at great depths, for example, 20,000 feet, in the sea.

Description of the prior art All transducers include a transducer means in the form of an oscillatory transducer element which when suitably energized projects an acoustic beam, and conversely when acoustic energy of the proper frequency or frequency range is received a corresponding electrical output signal will be provided by the transducer element. Such transducer elements are generally backed by a pressure release material which, in essence, simulates an air backing so that maximum acoustic energy is radiated from the front radiating surface of the transducer element. A variety of pressure release materials may be used among them being foam epoxies, sponge rubber, and a form of cork and neoprene rubber composition known as corprene to name a few.

The transducer element is placed in a housing with a covering member and a transducer fluid, such as high grade castor oil, is introduced in order to equalize and distribute static pressures which may be encountered in the sea. At depths of several hundred feet such a transducer construction provides satisfactory results in maintaining its desired beam pattern for intended operation. At greater depths however, for example thousands of feet, the pressure release backing due to the Water pressure transmitted thereto through the covering member and transducer fluid becomes compressed to a point where all of the air is crushed out of the pressure release backing material thereby destroying the intended transducer operation.

For deep sea operation which eliminates the need of a pressure release backing there has been proposed transducers which utilize a sound absorbing material having the same characteristics of the sea water so that energy radiated from the sides and back of the transducer element is substantially absorbed and the desired beam characteristic is maintained. This type of construction is satisfactory when operation is at elevated frequencies, for example, in the several hundred kilohertz (kHz.) range. At the lower frequencies however, for example below 100 kHz., the amount of sound absorbing material needed to do a satisfactory job can become prohibitive or impractical with present day sound absorbing mediums.

Patented Nov. 25, 1969 ice SUMMARY OF THE INVENTION A transducer according to the present invention includes an oscillatory transducer means which is operable at one or more frequencies within a certain frequency range and includes front, rear and side surfaces. An inertia loading mass extends around the back and up the sides at a distance d from the rear and side surfaces with the distance d chosen to be equal to a quarter wavelength, or any odd multiple of a quarter wavelength of the operating frequency (or an operating frequency within the said range). Spaced between the transducer and the inertia loading mass is a pressure transmitting means, preferably of a compliant medium having a relatively low specific acoustic impedance.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates a transducer element which may be used in the present invention;

FIG. 2 represents a transmission line, to aid in an understanding of the present invention;

FIG. 3 is an embodiment of the present invention in cross-sectional isometric view;

FIG. 4 illustrates a transmission line analogy of the structure of FIG. 3;

FIG. 5 is another embodiment of the present invention in cross-sectional isometric View;

FIG. 6 illustrates a transmission line analogy of the structure of FIG. 5;

FIG. 7 is a graph illustrating the relationship between operating frequency and relative mechanical impedance associated with the embodiment of FIG. 3; and

FIG. 8 illustrates another embodiment of the invention, in cross-section and isometric view.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 illustrates an oscillatory transducer means which may be utilized herein. The oscillatory transducer means is a transducer element 10 which includes a front surface F, a rear surface R and a plurality of side surfaces of which one, S, is designated. In order to better understand the problem encountered in use, operation of the transducer element 10 will be described in brief. When energized at a suitable operating frequency, such as by application of an electrical signal to the front and rear surfaces F and R, the front surface P will oscillate up and down in the direction of the arrow at a velocity U. The same is true of the rear surface R. The side surfaces for example S, will correspondingly oscillate in and out at a velocity which is approximately equal to one-third U. If the element 10 is considered to be surrounded by water, acoustic energy will be radiated from the front, rear and side surfaces of the element.

The power radiated into the water from a surface is:

P=V Z (real) 1 where V is the velocity of the surface and Z (real) is the real component of mechanical impedance which is presented to that surface by an adjacent medium. The Z is in the form of a+jb where a is the real component and b is the imaginary, or reactive component, of impedance. Reactive power [V Z (imaginary)] is also produced due to the reactive component of impedance and this reactive power tends to detune the transducer element. The mechanical impedance is a function of the specific acoustic impedance adjacent the surface, the area of the surface and the thickness of the adjacent medium. As-

suming a water medium of infinite length (thickness), the power radiated by the front surface F is:

PF: UZPCWL 2) where C is the specific acoustic impedance of the medium (water) adjacent the surface, U is the velocity V of Equation 1 and WL is the area of the front surface. (p is the density of the medium or material and C is the speed of sound through that medium or material.) To a good approximation the power radiated out the side surface S is:

U P OLT (3) where U/ 3 is the velocity V of Equation 1 and LT is the side area. In order to provide a directed forward looking beam pattern it is necessary to substantially reduce the effects of the acoustic energy radiated from the side and rear surfaces of the transducer element 10. Examining Equation 3, the power radiated from the side may be reduced by reducing L, the length of the transducer element. Reducing L however, will also reduce the power which is radiated from the front surface F since the L term is common to both equations. Such reduction in radiated power from the front surface F is undesirable. Another way of reducing power out the side is to reduce T, however reduction of T will not allow operation at the lower frequencies. In the present invention radiation from the side and rear surfaces is substantially reduced by placing around the side and rear surfaces a backing means which effectively reduces the mechanical impedance presented to the surface by the reduction of the specific acoustic impedance, the C term in the above power equations, presented to the side and rear surfaces.

Before proceeding with the constructional details of the transducer, reference should now be made to FIG. 2 representing a transmission line of length d having a characteristic impedance of Z A mechanical physical system has a direct analogy in a corresponding electrical system and the transmission line of FIG. 2 is presented in order to better understand the mechanical concepts involved herein.

The impedance Z looking in at the terminals A, B relative to the characteristic impedance Z is given by the formula:

.Z Z2 1- 0 cot 0 4) Z0 cot 0 Z0 9 where Z is the impedance at terminals C, D and:

27rd T 5 If d is made equal to a quarter wavelength, that is, M 4,

0 in Equation 5 would be 1r/2 and Equation 4 would reduce to:

rearranging Equation 6 Z 2 =Z Z 1 (7) in Equation 7 Z will be relatively low if Z is low and Z made high as practical. Application of Equation 7 to a mechanical system by the analogy to mechanical impedance in place of electrical impedance will be discussed with respect to FIGURES 3 and 4 to which reference is now made.

The transducer of FIG. 3 includes an oscillatory transducer means such as the transducer element 10 of FIG. 1. Backing means for the transducer element 10 includes an inertia loading mass 12 which extends around the back, and up the side surfaces of the transducer element 10 and spaced at a distance d therefrom. In the embodiment of FIG. 3 the inertia loading mass 12 also forms the transducer housing. The inertia loading mass 12 may be of any dense material offering resistance to dynamic movement. One example of a material which may be utilized as the inertia loading mass 12 is metal such as aluminum, lead, brass, gold, etc. For those metals which have little sea Water corrosion resistance a suitable coating or metallic jacket may be provided thereon.

Interposed in the space between the transducer element 10 and the inertia loading mass 12 is a pressure transmitting means 14. In the construction of the transducer described herein it is preferable that the pressure transmitting means 14 be a compliant material having a relatively low specific acoustic impedance C). The term compliance is the reciprocal of stiffness, that is the displacement corresponding to the application of a specific force. One type of material which may be utilized is a butyl rubber composition, and to aid in the absorption of acoustic energy radiated from the rear and side surfaces of the transducer element 10 the butyl rubber composition may include metallic particles such as aluminum powder or flakes embedded therein thus forming a sound absorbing medium. By way of example a sound absorbing butyl rubber material having the embedded aluminum flakes exhibits a compliance of approximately 10" inches per inch per pound, and a specific acoustic impedance approximately equal to that of sea water, which is approximately l.5 l0 MKS units. By way of comparison, an inertia loading mass formed of aluminum exhibits a specific acoustic impedance of l7 l0 MKS units, over 11 times that of the sea water.

It is to be noted that when operation is at high frequencies such as the hundreds of kilohertz range the sound absorbing material described may be suflicient to prevent acoustic radiation from the rear and sides of the transducer; however, at the lower frequencies such as below kilohertz the amount of sound absorbing medium required becomes prohibitive.

During oscillatory operation of the transducer element 10 there may be produced in the material 14 an acoustic wave which travels towards the front surface F of the transducer element 10. In order to decouple this unwanted radiation from the surrounding water medium there is provided inertia loading masses 20 and 21 which therefore aid in maintaining a proper beam pattern.

The transducer is provided with a covering member 24 and the housing is filled with a transducer fluid 26, the covering member 24 and transducer fluid 26 both having approximately the same acoustic transmission characteristics as sea water, such materials being well known to those skilled in the art. Static forces encountered at depth are applied to the covering member 24 and transmitted through the transducer fluid 26 to the pressure transmitting means 14 which then balances the pressure within the transducer to that of the surrounding water medium.

The backing means including the inertia loading mass 12 and pressure transmitting means 14 of FIG. 3 cooperate in a manner to substantially reduce undesired acoustic propagation from the rear and sides of the transducer element 10 and consequently from the rear and sides of the transducer, and also keeps to a minimum reactive power tending to detune the transducer element. An important consideration in the fabrication of the backing means is the selection for the distance d. To accomplish the desired result the distance d is made equal to M4 where A is the wavelength of the operating frequency of the transducer. The manner in which the propagation reduction is accomplished may best be understood by now making additional reference to FIG. 4 which illustrates a transmission line analogy of a portion of the backing means of FIG. 3.

The line from terminals A, B to terminals C, D is analogous to the pressure transmitting means 14, which has been made equal to a quarter wavelength, and therefore a quarter wavelength transmission line (as in FIG. 2) is represented between terminals A, B and C, D.

Coupled to terminals C, D is an inductor 30 and an impedance 32 representing respectively the impedance of the inertia loading mass 12 and the impedance of the surrounding water medium load. The impedance looking in at terminals A, B of FIG. 4 represents the impedance presented to a side surface (or the rear surface) of the trans ducer element 10. From the power equation 1 it is seen that the lower the impedance presented, the lower will be the power radiated from that particular surface. If the impedance 32 of the water load is arbitrarily normalized to unity and it may be assumed that it is substantially resistive, then the impedance of the inductor representing the inertia loading mass 12, assuming approximately 1 inch thick aluminum and transducer operation at 60 kHz., will be in the order of 20. Referring back to FIG. 2 and Equation 7, it may be seen that the impedance at terminals C, D is equivalent to Z while the impedance at terminals A, B is equivalent to Z The fact that a quarter wavelength of pressure transmitting means 14 is utilized coupled with the relatively high Z impedance at terminals C, D cooperate to provide a relatively low Z at terminals A, B thereby reducing the power andaccordingly the acoustic energy propagated from the side and rear surfaces of the transducer element while maximizing the power radiated from the front surface F. That is, with'the specific acoustic impedance of the water being normalized to 1, and the value of inductance of the inductor 30 being therefore equal to approximately 20, then from Equation 7 the impedance at terminals A, B will be:

multiplying the numerator and denominator of Equation 8 by 1j20:

i:fl2 -L 22 1 401 401 (9) in Equation 9 Z is of the form a+jb where the real component a is 1/401 and the imaginary or reactive component b is The real component of mechanical impedance presented to the side and rear surfaces of transducer element 10 therefore is approximately that of the mechanical impedance (real component) presented to it, were it coupled directly to the water. Similarly, 'detuning of the transducer element is substantially kept to a minimum since the'reactive component of impedance contributing to reactive power is negligible.

FIG. 5 illustrates another embodiment wherein acoustic radiation from the side and rear surfaces of a transducer element is further reduced. The transducer of FIG. 5 includes a transducer element 10 and spaced from the rear and "side surfaces therefrom by a distance d is a first inertia loading mass 35 which may, for example, be of a lead composition. interposed between the transducer element 10 and the first inertia loading mass is a first pressure transmitting means 38 which may be identical to the pressure transmitting means 14 of FIG. 3. The backing means further includes a second inertia loading mass 42 which forms the transducer housing and may be aluminum. The second inertia loading mass 42 is spaced from the first inertia loading mass 35 by a distance of d and the space therebetween is filled with a second pressure transmitting means 46. In a manner similar to FIG. 3, the transducer of FIG. 5 includes inertia loading masses 49 and 50, covering member 52 and transducer fluid 54. The transmission line analogy for the backing means of the transducer in FIG. 5 is illustrated in FIG. 6. The impedance at terminals A, B is the impedance presented to a side or rear surface of transducer element 10 and is equivalent to the Z of FIG. 2. The transmission line from terminals A, B to C, D is a quarter wavelength long and is equivalent to the first pressure transmitting means 38. Inductor 57 is representative of the impedance of the first inertia loading mass 35 and the inductor 60 is representative of the impedance of the second inertia loading mass, or housing 42. Impedance 61 represents the water load impedance. The transmission line from terminals A, B to C, D is of a distance d and is representative of the second pressure transmitting means 46. The impedance looking in at terminals A, B toward C, D is governed by Equation 4 and depending upon the length d, the value of inductor 60, the impedance of the water load 61 (assumed to be normalized to unity) there will be present at terminals A, B a certain impedance Z illustrated in dotted lines. The impedance at terminal A, B therefore is governed by the value of impedance Z the inductance value of inductor 57 and Equation 7 since a quarter Wavelength transmission line is utilized. In general the higher the value of X the lower will be the impedance at terminals A, B.

Results similar to those demonstrated with respect to FIGS. 4 and 6 may be achieved by using odd multiples of quarter wavelength pressure transmitting means and accordingly in general the spacing d of the first inertia mass from the transducer element 10 in FIG. 3 and FIG. 5 would be where n is an odd integer.

In general the operating frequency of the transducer is selected and accordingly a quarter wavelength distance may be calculated and the backing means of the trans ducer fabricated accordingly with the thickness of the inertia mass or masses chosen in accordance with the desired side and rear sound attenuation (decibels). In actuality, the operating frequency may vary somewhat within a limited range. If the operating frequency is chosen by way of example to be 60 kilohertz then M4 would be equal to inch and with the first inertia mass spaced from the transducer element at a distance of inch a minimum mechanical impedance is provided. If operation should be increased to a higher frequency or decreased to a lower frequency the A inch spacing would no longer represent a quarter wavelength. The curve of FIG. 7 illustrates a typical relationship of impedance to operating frequency. The horizontal axis represents frequency and the vertical axis represents impedance Z relative to impedance of sea water Z By way of example, if the operating frequency of I the impedance curve 64 is a minimum at frequency f A theoretical construction for a. 20 decibel attenuation utilizing inch lead as an inertia mass and A inch of sound absorbing rubber would, at a frequency f of 60 kHz. yields a Z/Z of approximately 0.05. Although 4 inch would not be the quarter Wavelength for an increase or decrease of operating frequency there still is experienced satisfactory operation within a certain frequency range for example, defined by the designation FR which intersects the impedance curve 64 at points 65 and 66 and which points may represent a relatively impedance Z/Z of 2-3 times more, that is 0.1-0.15 in the example given, than the minimum at f If the frequency of operation is increased past point 66 the relative impedance increases to an undesirable amount until further operation at again results in a minimum relative impedance with a similar operating range PR The curve 64 may be similarly continued however, there are transition points beginning at the higher and lower frequencies than these illustrated by way of example. In summary of FIG. 7, the transducer may be designed to operate at a chosen frequency for which a quarter wavelength section may be determined and for which a minimum mechanical impedance results in accordance with the desired attenuation and the inertia loading mass utilized. The transducer, however, can operate at certain other frequencies with one or more frequency ranges to still provide, although not optimum, at least satisfactory results. The designed operating frequency falls within the range and accordingly the distance d is equal to where A is the wavelength of one of the frequencies and n is an odd integer.

In FIG. 8 there is illustrated another embodiment of the present invention which utilizes a different type of pressure transmitting means not illustrated in FIGS. 3 and 5. Only one inertia loading mass 68 is illustrated in FIG. 8 although it is to be understood that a plurality of such inertia loading masses such as illustrated in FIG. may be utilized. The distance between the side and rear surfaces of transducer element 10 and the inertia loading mass 68 is equal to a quarter wavelength and the space therebetween is filled with a fluid 71. Various types of silicon fluids are available which exhibit a specific acoustic impedance below that of the sea Water and in addition the use of a fluid allows for pressure balancing when used at depth. In order to center the transducer element 10 there is provided a plurality of spacer means 73, an example of such spacer means being undulating or corrugated metal, the corrugations being provided in order to insure a spring action to decouple the element 10 from the mass 68.

Although the present invention has been described with a certain degree of particularity it should be understood that the present disclosure has been made by way of example. An important consideration in the fabrication of the transducer is the selection of distance d, in conjunction with at least one inertia loading mass. The size or shape of the transducer element may vary according to desired use. If such element is, for example, discshaped then such element would have a front and rear surface in addition to a curved side surface; such curved side surface could be thought of as an infinite number of surfaces and hence the terminology, side surfaces, used herein is applicable to various shapes of transducer elements. It is apparent that various modifications and variations of the present invention are made possible in light of the above teachings.

I claim as my invention:

1. A transducer comprising:

(a) oscillatory transducer means for operation within a frequency range, and including front, rear and side surfaces;

(b) backing means including (1) an inertia loading mass extending around said back and up said side surface and spaced at a distance d therefrom,

(2) pressure transmitting means interposed in the space between said transducer means and said inertia loading mass;

(c) said distance d being equal to Where A is the wavelength of a frequency within said range and n is an odd integer.

2. A transducer according to claim 1 wherein:

(a) the pressure transmitting means is a compliant material having a specific acoustic impedance C approximately equal to that of the fluid medium in which the transducer operates.

3. A transducer according to claim 2 wherein:

(a) the compliant material has metallic particles embedded therein to form a sound absorbing medium.

4. A transducer according to claim 1 wherein:

(a) the pressure transmitting means includes a fluid.

5. A transducer according to claim 4 wherein:

(a) the pressure transmitting means includes spacer means contacting the transducer means and the inertia loading mass.

6. A transducer comprising:

(a) oscillatory transducer means for operation within a frequency range and including front, rear and side surfaces;

(b) a plurality of inertia loading masses each spaced from one another and from said transducer means and each extending around said back and up said side surfaces, each of said inertia loading masses having a specific acoustic impedance;

(0) a plurality of pressure transmitting means respectively interposed between said plurality of inertia loading masses, and between said transducer means and a first of said inertia loading masses, each of said pressure transmitting means having a lower specific acoustic impedance than those of said inertia loading masses.

7. A transducer according to claim 6 wherein:

(a) the space between the transducer means and the first inertia loading mass equals where A is the wavelength of a frequency within said range and n is an odd integer.

References Cited UNITED STATES PATENTS RODNEY D. BENNETT, JR., Primary Examiner BRIAN L. RIBANDO, Assistant Examiner 

